Laundry Habits: What's Normal For Towels and Sheets?

Earlier this week Grace talked about shared laundry etiquette which led to some pretty amusing stories in the comments. It reminded me of an ongoing debate I have with my boyfriend about how often sheets and towels should be washed. I think in his mind, with towels especially, you're always clean when you use them so why would you ever need to wash them? Obviously we differ:

Generally I wash the bedsheets every week, or every other if I get lazy. I also tend to take a shower before bed so I'm definitely clean. Towels I also wash about every week, and I just alternate between 2 during the week so I end up using each one about 3-4 times. Wikipedia says that that's 'normal' for the US. I know if someone else wasn't changing my boyfriend's sheets it would be a lot longer between washings, and don't even get me started on the towels.

How often do you wash sheets and towels? When do you consider them dirty?